The genus Staphylococcus includes at least 20 distinct species. (For a review, see Novick, R. P., The Staphylococcus as a Molecular Genetic System, Chapter 1, pgs. 1-37 in MOLECULAR BIOLOGY OF THE STAPHYLOCOCCI, R. Novick, Ed., VCH Publishers, New York (1990)). The species differ from one another by 80% or more, by hybridization kinetics, whereas strains within a species are at least 90% identical by the same measure.
The species Staphylococcus aureus is a gram-positive, facultatively aerobic, clump-forming cocci considered among the most virulent species of the genus.
Staphylococcus aureus is a ubiquitous pathogen. (See, for instance, Mims et al., MEDICAL MICROBIOLOGY, Mosby-Year Book Europe Limited, London, UK (1993)). It is an etiological agent of a variety of conditions, ranging in severity from mild to fatal. A few of the more common conditions caused by S. aureus infection are burns, cellulitis, eyelid infections, food poisoning, joint infections, neonatal conjunctivitis, osteomyelitis, skin infections, surgical wound infection, scalded skin syndrome and toxic shock syndrome. Staphylococcus aureus also causes more serious illnesses such as pneumonia, meningitis and bacteremia.
When introduced in food, Staphylococcus aureus may produce one or more Staphylococcal enterotoxins. If ingested, heat stable Staphylococcal enterotoxins may produce symptoms of food poisoning and a range of other diseases.
Staphylococcus aureus possesses a protective cell wall which comprises a cross-linked peptidoglycan layer. The cell wall is resistant to phagocytosis which is thought to be due, in part, to the production of Protein A on the cell surface. Staphylococcus aureus also produces hemolytic toxin which may damage blood cells and immune cells.
Accordingly, it is readily apparent that it is important that bacterial pathogens such as Staphylococcus aureus can be identified in samples. To that end, several methods have been disclosed for detecting Staphylococcus aureus. For example, U.S. Pat. No. 5,496,706 discloses a method for detecting Staphylococcus aureus in a sample. The method uses anti-MRSA-230 antibodies to detect Staphylococcus aureus by visible agglutination means. This method, though, can be quite expensive.
Many laboratories perform a slide coagulase test as a rapid, inexpensive method for presumptive identification of S. aureus. This test relies on the observation of clumping of a heavy suspension of S. aureus cells in the presence of a drop of plasma placed on a slide. The clumping is mediated by the binding of fibrinogen with a specific receptor of the surface of the bacterium, termed clumping factor. However, a slide coagulase test has significant drawbacks. For example, 10 to 15% of S. aureus isolates yield false negative results requiring all negatives to be confirmed in a tube coagulase test. A tube coagulase test is another commonly used method for identifying S. aureus. This method, though, requires an incubation period of 4 to 24 hours.
In addition, rapid cycle, real-time PCR methods can provide results in thirty (30) minutes and in situ probe hybridization methods such as FISH can provide results within ninety (90) minutes. However, these methods are expensive and/or labor intensive.
Further, testing of positive blood culture broth samples using latex tests has not been recommended due to poor assay sensitivity and specificity. See McDonald, C. L. et al., J. Clin. Microbiol. 33:50-52 (1995) and Spears, D. J. et al., J. Clin. Microbiol. 36:1032-1034 (1998). Thus, there is a need in the art for a rapid, accurate and inexpensive means for detecting and identifying bacterial pathogens including Staphylococcus aureus. 